To enhance or increase the production of oil and gas hydrocarbons from wells bored into subterranean-formations, it has been common practice to pump a viscous fluid at high pressures down in to the well bore to crack the formation and force the fracturing fluid into those cracks. The fracturing fluid is also used to carry sand or other types of particles, called proppants, to hold the cracks open when the pressure is relieved. The cracks held open by the proppant provide additional paths for the oil or gas to reach the wellbore, which increases production from the well.
Because of the high volumes of fracturing fluids used, it is desirable to thicken the fracturing fluids with very efficient thickeners. Efficient thickeners such as guar gum are commonly used. The viscosity of solutions of guar gum and similar thickeners can be greatly enhanced by crosslinking them with boric acid or other boron containing materials. Thus, boron crosslinked guar gum solutions are useful as fracturing fluids.
Continuous processes that allow the fluids to be made in “real time” during the fracturing process have a number of advantages over a batch process of mixing water, gelling agents, or other additives into individual “frac” tanks before treatment has begun. The batch process is expensive because of the time and equipment required because of wasted and unused fluids resulting from treatment delays, termination of treatment before pumping of fluids, and fluid left in the bottom of the tanks which cannot be pumped out.
To achieve the highest amount of down whole viscosity in a boron crosslinked fracturing fluid, conventional wisdom has held that the guar gum or similar thickener should be completely hydrated prior to addition of the crosslinking agent. In general, to hydrate a polymer, the particles of polymers must first be dispersed in water so its individual particles can absorb water. The polymer's ability to absorb water dictates the hydration rate, which is affected by the temperature, the fluid, the shearing energy added during hydration, and the like. Hydration of the polymer over time is seen by an asymptotic increase of viscosity to a maximum over several minutes up to an hour or more depending on temperature, concentration, and other factors. A process for producing a boron crosslinked fracturing fluid involves continuously dispersing the polymer in water and hydrating the polymer by holding it for the required time, while applying required shear energy to reach its final linear viscosity. When full hydration is reached, the boron crosslinker is added and the fracturing fluid is pumped down the wellbore to induce fracturing in the subterranean formation.
The equipment used to continuously mix and hydrate polymers in the field consists of a mixing section where the polymer is dispersed in water and a hydration section, which is generally a multi-compartment tank of large volume designed to maintain a first in/first (FIFO) out flow pattern to allow the time required to complete hydration of the polymer before exiting the unit. To reach full hydration, the residence time in the hydration system needs to be sufficiently long to achieve a minimum viscosity before the boron crosslinker is added.
The size of the mixing equipment is dictated by the rate at which finished fluid is needed and the time to achieve proper hydration before addition of the crosslinker. For example, traditionally, where the targeted fracturing fluid delivery rate is from about 20 to about 70 barrels per minute, a typical volume of the hydration section is on the order of 250 barrels or greater. To a certain degree money can be saved by providing smaller hydration equipment if a lower pumping rate is used. Generally, this is not desirable because it reduces the efficiency of the fracturing process.
A process for continuously producing fracturing fluids in real time that could be carried out with significantly smaller equipment such as mixing units would be a significant advance in the art. Not only would such equipment be less expensive than the conventional process, but a downsized fracturing fluid production system could be used at high pumping rates even in hydrocarbon producing locations where the infrastructure does not support use of the large conventional equipment.